1. Field of the Invention
The present invention relates generally to the processing of modulated data signals and, in particular, to equalization of such signals which have been transmitted over a channel which introduces both linear and non-linear distortion.
2. Description of the Prior Art
When high-speed data signals are transmitted over limited bandwidth, e.g., switched voiceband telephone transmission channels, various channel impairments give rise to distortion which in turn results in a phenomenon known as intersymbol interference. This phenomenon is a manifestation of the fact that a pulse passing through a distorted band-limited channel expands in the time domain. As a result, each sample of the received signal is not simply derived from a single transmitted data symbol but, rather, some combination of symbols. Impairments are caused by phase jitter, additive noise and non-flat frequency response in the channel, and result in amplitude and delay distortion which is often characterized as linear or non-linear.
Intersymbol interference which results from linear distortion is manifested in that each sample of the received signal contains a linear combination of a transmitted symbol which the sample nominally represents with symbols which precede and succeed it in the data stream. Known techniques which compensate for linear distortion in both baseband and passband have been quite successful, and include linear feedforward equalization and linear decision feedback equalization. In accordance with the former technique, each sample of the received signal is added to a weighted linear sum of past and future samples, prior to a decision being made as to the value of the transmitted symbol. In accordance with the latter technique, a weighted linear sum of past decisions is added to each sample, again prior to a decision being made as to the value of the transmitted symbol. See, for example, U.S. Pat. No. 3,974,449 issued to D. D. Falconer on Aug. 10, 1976.
Intersymbol interference caused by non-linear distortion (sometimes called "harmonic distortion") is manifested in that each sample of the received signal represents a combination of products of the current, past and future modulated data symbols, and/or the complex conjugates of such data symbols in systems that employ quadrature amplitude modulation (QAM). In transmission systems that employ linear modulation, such as QAM, the effect of non-linear distortion is to reduce the margin against noise. Indeed, for data rates above 4800 bps, which are needed in order to provide new services such as digitized encrypted speech, high-speed facsimile and high-speed dialed backup capability, non-linear distortion is an important impairment on many voiceband channels. Attempts to compensate for non-linear distortion, while somewhat successful, have nevertheless not been fully effective, for a variety of reasons. For example, the arrangement in U.S. Pat. No. 3,600,681 issued Aug. 17, 1971 to T. Arbuckle compensates for non-linear intersymbol interference only in baseband data signals. U.S. Pat. Nos. 4,213,095 and 4,181,888 issued to D. D. Falconer on July 15, 1980 and Jan. 1, 1980, respectively, describe separate techniques for feedforward and feedback non-linear equalization of modulated data signals. Even if the Falconer apparatus is combined to yield both feedforward and feedback non-linear equalization, the resulting arrangement, shown in FIG. 3(a) of an article by Falconer entitled "Adaptive Equalization of Channel Nonlinearities in QAM Data Transmission System" BSTJ, Vol. 57, pp. 2589-2611, (1978) is not well suited for economical, real-time practical implementation. First, the accuracy of the replica of channel nonlinearity (Y.sub.NL (n)) that is constructed is impaired by the fact that the apparatus is driven by noisy and distorted received samples R(n). Second, the number of registers needed to store the values of received samples R(n) that are used in the calculation of each correction term is quite high. Third, the Falconer architecture requires that the same timing phase be used for both linear and nonlinear equalization, and such an arrangement may not be optimum. Finally, the prior art arrangement is not amenable to use as a "plug-in" or "add-on" to a conventional linear receiver (as found, for example in the AT&T 2096A data set) since error values formed using the decision output of the non-linear equalizer must be used to update the coefficients contained in the linear equalizer.
In view of the foregoing, it is the broad object of the present invention to provide a method and arrangement which compensates for non-linear distortion which occurs when a modulated data signal is transmitted via a limited bandwidth channel. Specific objects are to efficiently compensate for intersymbol interference by constructing an accurate replica of the channel nonlinearity, where the required calculations can be performed in real time without exceedingly complex hardware requirements. In addition, elimination of timing phase problems and compatibility of the present invention with commercially available linear processors are also desired.